The Mastery system is a new game mechanic added to the talent system in 0400World of Warcraft: Cataclysm. It has undergone extensive revision since its announcement at Blizzcon 2009. With the dramatic revamp of the talent system in preparation for 0500Mists of Pandaria, mastery now represents a general bonus from items that gets applied to specialization (chosen at level 10) as single "Mastery: " ability which determines how the bonus is converted to some benefit.

Contents

The mechanic hasn't changed dramatically, but theoretically, the system is much simpler since the benefit from mastery is clearly stated in the single specific ability gain from choosing a specialization.

Your mastery is a passive benefit to your character based on what talent specialization you chose. As noted in the tooltip for each paladin mastery here on Wowhead, you can increase the effectiveness of your mastery bonus by stacking items with mastery rating: http://www.wowhead.com/talent#s

Some classes or talent specializations desire mastery rating much more than others, depending on how powerful the bonus is to their role, but the basic idea is that this passive skill is attractive to you because of the talent specialization you chose.

This is used in items with a level requirement of 78 or above[1] or item level 272 and up.[2] For example some uncommon items will have mastery by default, just like stamina or intellect.
Mastery is in addition to talent trees, this helps the Mastery buff gained at level 80.

Basically, the Mastery rating means nothing unless you have the appropriate Specialization which is a new concept introduced in 0400 Cataclysm. Specialization is basically a form of locking in your talent tree and doesn't happen until level 80.